Wavelength-routed (WR) networks are one of the important networking infrastructures to provide the required transmission bandwidth for the rapidly increasing Internet traffic. In WR networks, wavelength division multiplexing (WDM) divides the transmission bandwidth of optical fiber into many, if not hundreds of wavelength channels. Two users desiring communication can set up a lightpath connection by simply reserving a wavelength channel on each fiber link of the path between them. Traditionally, all wavelength channels have been allocated the same amount of bandwidth for simplifying and standardizing the implementation and deployment, e.g., the 100 GHz frequency (0.8 nm wavelength) spacing in ITU grids.
As transmission technologies advance, wavelength channels will often be under-utilized, i.e. channels are over-provisioned for normal user traffic. To have a better bandwidth utilization, efforts have been made on packing more low data traffic into a wavelength channel, using a smaller channel spacing such as 50 and 25 GHz, and more recently, using the variable bandwidth allocation of wavelength channels. While the importance of properly matching channel bandwidth to users' demand has been widely recognized, the mismatch between the ratio of the capacities (numbers of channels) deployed in the two transmission directions of a fiber link has been overlooked. For example, even though a new lightpath request is blocked because of no idle wavelength channel being free in a WR network at the required transmission direction, it is possible that some are available in the other transmission direction.